In many situations, devices must operate in potentially hazardous conditions, such as where a fuel mixture may be ignited by uncontrolled operating or environmental conditions. For example, vehicles, including aerospace vehicles, typically operate with a fuel that must be maintained in a safe condition during storage and use. The ignition hazard should be minimized even when the vehicle is subject to uncontrolled events such as an accident, electrical malfunction, a lightning strike, or static electrical discharge. Other applications requiring ignition hazard consideration include fuel transport, fuel storage, mining operations, chemical processing, power plant construction and operation, and operations which involve combustible particulate such as sawdust, flour, and grain.
In the aerospace industry, lightning strikes of aircraft are a concern because they could result in electrical arcs and/or heating sufficient to ignite vaporous fuel mixtures. Though lightning passes through metallic aircraft virtually always without resulting harm, manufacturers and regulators are vigilant to the potential for harm. The Federal Aviation Authority (FAA) recognizes that arc energies as low as 200 μJ (microjoules) may be sufficient to cause unwanted ignition of vaporous fuels.
Design of apparatuses exposed to ignition hazards typically involves reducing the likelihood of ignition, containing the ignition hazard, and/or withstanding the ignition hazard. Test systems to facilitate and/or verify such designs typically use a test article, which is a model of the apparatus or a component of the apparatus, and subject the test article to an energy discharge that simulates and/or generates the potential hazard. For example, test systems may include a lightning strike simulator.
Test systems may determine the presence and/or effects of generated ignition sources by surrounding (and/or filling) a test article with a flammable gaseous mixture. If an ignition source is generated, the flammable mixture burns and the resulting light, sound, heat, pressure, etc. may be observed. Particularly important for this flammable mixture technique, the test article is enclosed by a test chamber that is configured to contain the flammable gas and resulting (potentially explosive) ignition. In part due to the safety concerns of working with large quantities of flammable gases, this technique requires specialized equipment, facilities, and highly trained personnel (and thus is costly and cumbersome).
The flammable mixture technique may be performed generally following one of two approaches: an open flow-through approach and a closed system approach. In the open flow-through approach, a test chamber is designed and built for the specific test article, mass flow controllers are used to flow specific ratios of gases to make a controlled flammable mixture within and/or around the test article, an environmental condition is induced on the test article, and then a pass/fail criteria is designated from the resulting no-ignition/ignition of the flammable environment in the test chamber. Inaccuracies in the flammable mixture composition, delivery, and flow yield uncertainties in the test results (small changes in the gas concentration results in a relatively large change in ignition threshold sensitivity). The uncertainty in the test forces an increased number of (typically costly) tests of (typically costly) test articles. Likewise, the validation process for this test approach is cumbersome and time consuming.
The closed system approach uses a closed/vacuum tight test chamber in which the chamber is pumped out and filled using the method of partial pressures to achieve highly accurate flammable gas mixtures (thus achieving consistent ignition sensitivity). The drawbacks to this approach include expensive test chambers, limited test article and test chamber sizes (due to both safety and cost), long test setup times (due to pumping and filling), and long preparation times (due to test chamber fabrication).
Further, design of large and complex apparatuses would be facilitated by testing larger and/or more representative test articles. However, testing larger articles with conventional techniques involves consequently larger test chambers and/or larger amounts of combustible material (such as fuel in the test article and/or flammable mixtures to detect ignition sources).